The WO 2008/155716 discloses an optical biosensor in which an input light beam is totally internally reflected and the resulting output light beam is detected and evaluated with respect to the amount of target components at the reflection surface. When a plurality of analytes shall be tested in parallel in such or a similar biosensor, it is often necessary to provide the sample in spatially separated chambers. This increases the field of view that has to be surveyed, leading either to image deteriorations at the periphery of the imaged area or to complex optical systems.